Total Kjeldahl Nitrogen (TKN) analysis measures organic and inorganic forms of nitrogen and determines the protein content of food products. The analysis starts with the acid digestion of the sample, converting organic nitrogen to ammonia. This requires boiling the sample in concentrated sulfuric acid, potassium sulfate, and a copper catalyst to convert the organic nitrogen to ammonia.
The second part of the method involves the steam distillation of the digestate described above. The pH of the acidic digestion sample must be raised to 9.5 by adding concentrated sodium hydroxide. At this pH value ammonia gas forms; this is transferred by steam-distillation into the acidic absorbing solution, namely diluted boric acid, where it gets converted to ammonium. Nitrogen concentrations within the receiving solution can then be analyzed using classical potentiometric or colorimetric determination. This method and its various forms of quantitative nitrogen analysis within the collected distillate are well documented in compendiums from the EPA,AOAC, AACC, ASTM.
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